Piezoelectric oscillator



Oct. 18, 1949. P. D. GERBER PIEZOELECTRIC OSCILLATOR Filed May 29, 1947 INVENTOR. B401 12 652552 ATTORNEY Patented Oct. 18, 1949 PIEZOELECTRIC OSCILLATOR Paul D. Gerber, Woodlynne, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application May 29, 1947, Serial No. 751,383

6 Claims. 1

My present invention relates to piezoelectric oscillators and particularly to improvements in oscillating circuits and systems wherein the crystal is mounted, without any clamping force, between two or more spaced apart electrode plates.

Piezo-electric crystals that have been cut to oscillate at a frequency lower than 2000 kilocycles per second are extremely sensitive to any force which tends to limit their freedom of movement. Accordingly, the prior art dictates the use of socalled simple air-gap holders for the mounting of such crystals, and many ingenious ways have been devised for minimizing even the small damping effect necessarily present in such holders due (a) to friction between the crystal and the electrode surface uponwhich the crystal rests (see Smalts 2,231,483 and Gerber 2,284,088) and (1)) due to the presence of the pins which limit the extent of the lateral movement of the crystal upon the said electrode surface (as to this see, Clark 2,139,998 and Bokovoy 2,285,143).

In spite of the foregoing mechanical precautions, and in spite of certain improved electrical features of construction (e. g. Smalts 2,139,918), incidences have arisen wherein air-gap mounted crystals failed to exhibit the stability of performance required in the operation of oscillators designed for use in radio broadcast and other systems for the communication of intelligence. In

studying such unsatisfactory crystal units I have discovered that their breakdown or faulty performance resulted from, or was accompanied by, vertical displacement of the crystal in its air-gap. In no case could this vertical movement be attributed to shocks or tremors of external origin and I have traced the fundamental cause of the said undesired movement of the crystal to the accumulation of static charges of opposite polarity upon the u per electrode of the holder and upon that face of the crystal which is presented (across the air-gap) to the said electrode.

Accordingly, the principal object of my invention is to provide a method of, and means for, preventing or reducing to a negligible amount, the accumulation of static charges in crystal oscillator installations of the type wherein the crystal is contained in a so called air-gap" type of holder.

My invention will be described in connection with the accompanying drawing wherein Fig. 1 is a side-elevational view of a conventional airgap holder containing a piezoelectric crystal; certain parts being marked with electrical symbols which will be referred to in describing the problem here involved, and Fig. 2 is an, electrical 2 diagram showing a crystal controlled oscillator incorporating my invention.

As previously indicated, the quartz or other piezoelectric crystal X employed for controlling the frequency of a radio broadcast or other oscillating circuit operating at a frequency of less than 2000 kc. is almost invariably placed without clamping pressure upon the bottom electrode B of a simple air-gap holder H having retaining pins l, 2, 3 and 4 positioned, say, adjacent two diagonally opposite corners of the crystal, for limiting the degree of lateral movement of the crystal. It can be demonstrated that when such a crystal unit is connected to a direct-current potential (applied through the leads a and D) such that the upper electrode A is positive and the lower electrode B is negative in potential, then, the top electrode A will immediately acquire numerous positive charges and the lower electrode B negative charges, with a resulting force of attraction between them. Given time, some negative charges will appear on the top surface of the crystal as if in an eiTort to effect a conducting circuit external to the source of potential difference. Because of the dielectric and insulating properties of'the crystal, and the relatively small air-gap thereabove, it is difilcult exactly to visualize the distribution of the said electrical charges. However, eventually these charges appear on the top surface of the crystal in a quantity suflicient to yield a force of attraction great enough to overcome the gravitational pull on the crystal. Asa result, the crystal will be lifted from the bottom electrode and produce a jump or shift in operating frequency. If by contact with the top electrode, or by interruption of operation, the charges are neutralized or otherwise removed from the top surface of the crystal so that it drops back into normal operating position on the lower electrode, the above described operating cycle will be repeated. It is of course apparent that the same phenomenon can also take place with the D.-C. potential reversed in polarity and, it is logical to assume,

with crystals other than quartz.

In many present day crystal controlled broadcast transmitters with which I am familiar the crystal unit is subjected to a direct-current potential capable of producing the above described unstable operating conditions. At first glance it might appear that the undesirable behavior of the crystal might be cured by reducing the D.-C. potential applied to the vacuum tube but this is precluded, at least in the broadcast field, by the fact that the power output of some if not all of the many different types of oscillators would be impaired by such a direct procedure. It is thus apparent that the desired result must be achieved without adversely affecting either the vibratile performance of the crystal or of the oscillating circuit which is connected thereto at the points a and b.

The foregoing and other requirements are met in accordance with my intention by introducing a resistance-capacitive (R.-C.) network in the circuit of the oscillator on the side of the terminals a and b which lies adjacent (electrically) to the crystal unit, as shown in Fig; 2. This network, which may be incorporated in any conventional crystal controlled oscillator, is shown as comprising a blocking condenser C in series with the crystal and a leakage resistor R. in shunt with the crystal. With the capacitor C and resistor R, thus arranged the D.-C. potential present across the terminals a and b will be divided between R and C as determined b the ratio of the leakage resistance of the condenser C and the ohmic resistance of the resistor B. (This assumes that the D.-C. resistance of the air-gap is substantially infinite.) This ratio:

C (leakage resistance) R (rated ohmic resistance) for a specific magnitude of D.-C. potential between a and b must be sufficiently high to prevent an accumulation of charges on the upper electrode (and on the adjacent face of the crystal) suflicient to lift the crystal off the bottom electrode. The leakage resistance of the well designed condensers now commercially available is at least 1000 megohms, and when R comprises a 10 megohm resistor the resulting ratio of 100 to 1, or higher, has proved entirely satisfactory for present day broadcast oscillator circuits.

My invention may be applied to crystal controlled oscillators of various types, irrespective of whether the D.-C. potential to which the crystal is subjected is derived from a grid resistor, directly from the plate current source, or from both. In the particular embodiment of my invention which has been selected for purposes of illustration the electrodes A and B for the crystal X are connected respectively to the anode 5 through the capacitor C, and to the grid 6 of a three element vacuum tube 1. The anode 5 is connected to the positive terminal (-1-) of a source of direct current through a tank circuit 8, and there is a grid resistor 9 connected between cathode l0 and grid 6. In this case the direct current potential to which the crystal is subjected comprises a combination of the D.-C. potential from the anode supply, less the D.-C. potential developed across the grid resistor 9. In one typical case wherein the D.-C. potential applied to the anode was 165 volts and the potential across the grid resistor 9 was 54 volts the net potential between the anode and grid was 111 volts. An appropriate value for the blocking condenser C in this case was 6500 mmfd. and the value of the leakage resistor R was of the order of 10 megohms. Thus, in agreement with the formula when the leakage resistance of the capacitor C was of the order of 1000 megohms the D.-C.vo1tage appearing across the crystal was substantially 1 percent of 111 volts or 1+volts. This relatively minute D.-C. voltage across the spaced apart electrodes A and B of the air-gap holder H did not permit the accumulation of static charges of suflioient magnitude to lift the crystal off its bottom electrode and hence stabilize the vibratile performance of the crystal and the output of the oscillator.

I claim as my invention:

.1. The combination with a piezoelectric oscillator'of the type wherein the piezoelectric element is normally subjected to direct current potentials, of a capacitor having one terminal connected to one side of said piezoelectric element and the other terminal connected to a point in the oscillatory circuit common to said D.-C. potentials thereby effectively isolating all said D.-C. potentials from said piezoelectric element except for potentials due to leakage through said capacitor, and a resistor connected in shunt with on y said piezoelectric element to provide a path for said leakage thereby minimizing the accumulation of charge on said element.

2. The invention as set forth in claim 1 and wherein said oscillator comprises a vacuum tube having a grid resistor in circuit therewith and wherein said direct current potential is derived at least in part from said grid resistor.

3. The invention as set forth in. claim 1 and wherein said oscillator comprises a vacuum tube having an anode and a source of energizing current connected to said anode, and wherein said direct current potential is derived at least in part from said anode supply.

4. The invention as set forth in claim 1 and wherein the leakage resistance of said capacitor is substantially no less than 1000 megohms and the resistance of said resistor is of the order of 10 megohms.

5. The invention as set forth in claim 1 and wherein said piezoelectric element comprises a quartz crystal cut to respond to a frequency substantially no higher than 2000 kilocycles per second and wherein said quartz crystal is mounted in an air-gap type of holder.

6. The invention as set forth in claim 1 and wherein the ratio of the leakage resistance of said condenser to that of the ohmic resistance of said resistor is sufficiently high to prevent the accumulation of static charges in said holder.

PAUL D. GERBER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,064,013 Kinn Dec. 15, 1936 2,113,210 Hight Apr. 5, 1938 2,137,687 Hanseli Nov. 22, 1938 2,298,774 Parker Oct. 13, 1942 2,411,765 Usselman et al Nov. 26, 1946 

